Thanks for your fast reply. Your point it well taken about the problem of a seller doing the FDE.
The reason I dread doing a fresh Ubuntu reinstall myself is because when I tried it over a year ago with a laptop that was originally Windows, the Ubuntu install didn't go well at all - Ubuntu just wouldn't boot. I tried a single Ubuntu OS install, not dual boot. I recall vaguely the difficulties had to do with the Ubuntu boot file(s) not being in the right place on the drive, or some such. After tons of difficult troubleshooting I eventually got Ubuntu to boot, but it never did boot normally, or easily.
Would you say it's likely that a fresh reinstall would go more smoothly on a laptop that was initially Ubuntu, rather than one that was originally Windows? I'd feel much better about doing the reinstall if that were the case. Or does it not matter which OS you're installing on top of (as far as likelihood of problems)?